The present invention relates to a low-noise preamplifier stage, in particular for magnetic heads. More in particular, the present invention relates to a preamplifier stage which has a ground-connectable input and can be directly coupled to the magnetic head.
Generic preamplifier stages in various configurations are known. However, none of these configurations fully and simultaneously meets all the requirements of the specific application.
For example, in the development of a preamplifier stage for the described application, a configuration which comprises an output stage provided via an NPN transistor driven by a single-transistor stage, also of the NPN type, has been studied. A possible circuital solution based on this configuration is shown in FIG. 1. In this figure, T.sub.2 indicates the output transistor and T.sub.1 indicates the input transistor, R.sub.1 and R.sub.2 are used to define the value of the total gain of the stage, and V.sub.REF is the bias voltage required to appropriately bias the output (due to the single-transistor structure of the output stage), such auxiliary voltage being applied to the base of the transistor T.sub.1 via resistor R. The capacitor C provides an alternative-current path through which the input signal can be applied, and has therefore a terminal connected to the base of T.sub.1 and the other terminal arranged so as to define the input IN of the stage.
This stage meets the requirement of low noise, since the total noise of the stage is predominantly due to a single transistor, i.e. to T.sub.1, whereas the noise due to the transistor T.sub.2 can be ignored, since it is returned to the input divided by the amplification of the first stage (which includes T.sub.1).
However, this stage is not ground-connectable, cannot be directly coupled and furthermore has additional problems related to the assignment of the values of R and C. R and C in fact must be given values so that the virtual high-pass filter which they form allows the passage of the audio signal V.sub.IN applied to the input, i.e. so that the time constant EQU .tau.=R.multidot.C
of said filter is large enough. However, R cannot have a high value in order to avoid deteriorating the stage input noise. For a given value of the time constant, it is therefore necessary to choose a high capacity value C, and this causes a considerable increase in the cost of the overall system.
In order to overcome the above described problems, a different configuration, which uses a differential stage for the input stage, has been developed. The related circuital solution is shown in FIG. 2, in which T.sub.1 again indicates the input transistor, R.sub.1 and R.sub.2 again indicate the resistors which set the gain of the stage, and the differential stage is formed by two identical PNP transistors indicated by T.sub.3, T.sub.4. In particular, the two coupled emitters of T.sub.3, T.sub.4 are connected to a current source, indicated by 2I, and their collectors are connected to the ground via respective resistors R.sub.3, R.sub.4. The base of T.sub.4 is connected to the intermediate point between R.sub.1 and R.sub.2, whereas the base of T.sub.3 is connected to an inductor L which represents the magnetic head, the other terminal whereof must be connected to a reference voltage V.sub.REF.
A direct coupling is therefore obtained in this solution, but it is still necessary to provide for an auxiliary voltage source to obtain the voltage V.sub.REF required to bias the output to a value higher than that defined by the ground. This entails the known problems related to greater structural complication, to the presence of drifts, etc. Furthermore and most of all, this structure entails a considerable deterioration of the noise performance with respect to the circuit of FIG. 1. In fact, by setting T.sub.3 and T.sub.4 to an equal current level and optimizing the structure for the source resistance, indicating the input equivalent noise source of T.sub.3 with e.sub.n1, and assuming, for the sake of simplicity, that only the transistors produce noise, the total noise of the stage is: ##EQU1##